1. Field of the Invention
The present invention relates to a secondary beat signal cancel circuit suitable for use in a VTR (video tape recorder) in which a low-frequency converted carrier chrominance signal is recorded together with a luminance signal on a magnetic tape.
2. Description of the Prior Art
In a VTR (video tape recorder) in which a carrier chrominance signal is converted to a low frequency and then recorded together with a luminance signal on a magnetic tape, the tape - rotary magnetic head system has a non-linear characteristic, namely a tertiary curve characteristic, so that, as shown in FIG. 1, a secondary beat signal component C.sub.B of the low-frequency converted carrier chrominance signal C leaks into the luminance signal Y. The signals are recorded on the magnetic tape in that state. Upon reproducing, because of the secondary beat signal component C.sub.B, an oblique beat pattern appears on the picture screen and thereby the picture quality is deteriorated greatly. Particularly when the level of the chroma signal is large, the deterioration of the picture quality is substantial.
Therefore, in the prior art, a reproducing system of the VTR is generally provided with a secondary beat signal cancel circuit which cancels the secondary beat signal component C.sub.B. FIG. 2 is a circuit block diagram showing an example of such prior art secondary beat signal cancel circuit.
In FIG. 2, a luminance signal Y and a low-frequency converted carrier chrominance signal C which are reproduced by a magnetic head H are supplied through a pre-amplifier 1 to a band pass filter 2 by which the low-frequency converted carrier chrominance signal C is separated from the luminance signal Y. This low-frequency converted carrier chrominance signal C is supplied to a frequency converting circuit 3 and then fed to a demodulator 4 which generates a pair of component chrominance signals, for example, red and blue color difference signals R - Y and B - Y.
The output signal from the pre-amplifier 1 is further supplied to a high pass filter 6 by which the luminance signal Y is separated from the carrier chrominance signal C. The luminance signal Y is then supplied to a demodulator 7 in which it is FM (frequency modulation) - demodulated. Thereafter, the FM-demodulated luminance signal Y is supplied to a secondary beat signal cancel circuit 10 in which the secondary beat signal component C.sub.B contained in the luminance signal Y is cancelled out. Thus, the luminance signal Y having no secondary beat signal component C.sub.B is delivered to a terminal 8.
The secondary beat signal cancel circuit 10 includes a non-correlation signal detecting circuit 11. The non-correlation signal detecting circuit 11 consists of a delay circuit 12 supplied with the output of the demodulator 7 and delaying the same by one horizontal period (1H), an inverter 13 supplied with the output of the delay circuit 12 and an adding circuit 14 which adds the present luminance signal from the demodulator 7 and the luminance signal delayed by 1H from the inverter 13. Accordingly, from the adding circuit 14 a non-correlation signal is derived which includes the secondary beat signal component C.sub.B contained in the luminance signal Y and an edge signal .DELTA.Y (a signal corresponding to an edge portion of the luminance signal Y) upon vertical correlation. When the vertical correlation exists, no edge signal .DELTA.Y is produced.
The non-correlation signal formed of the secondary beat signal component C.sub.B and the edge signal .DELTA.Y is supplied through a band pass filter 15 to a limiting amplifier 16 which limits the portion of the edge signal .DELTA.Y having a level higher than a predetermined level. The non-correlation signal which is limited as above is supplied to a level control circuit 17 in which the level thereof is controlled by an envelope-detected output of the carrier chrominance signal produced from an envelope detecting circuit 19 which is supplied with the carrier chrominance signal C from the band pass filter 2. Then, the non-correlation signal from the level control circuit 17 is supplied to a subtracting circuit 18 in which the non-correlation signal is added to the present luminance signal from the demodulator 7 with polarities as shown in FIG. 2. Reference numeral 30 denotes a delay element which is connected between the demodulator 7 and the subtracting circuit 18 to compensate for the delay time caused by the interposition of the band pass filter 15, the limiting amplifier 16 and so on.
The level control circuit 17 is adapted to control the level of the non-correlation signal to become such a level as to cancel the secondary beat signal component C.sub.B contained in the luminance signal Y.
When the vertical correlation exists, no edge signal .DELTA.Y is produced from the adding circuit 14 so that at this time, the level control circuit 17 produces only the secondary beat signal component C.sub.B the level of which, is controlled on the basis of the envelope-detected output; thus the edge portion of the present luminance signal is not deteriorated at all.
However, when a color picture image of an oblique stripe pattern as, for example, shown in FIG. 3 is recorded, no vertical correlation exists between the edge portions of two succeeding horizontal lines, for example, n and n+1 so that in this case, the edge signal .DELTA.Y is also produced together with the secondary beat signal component C.sub.B. Then, the edge signal .DELTA.Y is controlled in level on the basis of the envelope-detected output too, so that the edge signal .DELTA.Y is subtracted from the present luminance signal Y and thus the edge portion of the present luminance signal Y is deteriorated.
When the color picture image as, for example, shown in FIG. 3 is recorded, a signal as shown in FIG. 4B is produced as the present luminance signal Y on the horizontal line, for example n+1, and the 1H delay circuit 12 produces the luminance signal Y of the horizontal line n (FIG. 4A). As a result, from the adding circuit 14 a secondary beat signal C.sub.B is derived containing an edge signal .DELTA.Y as shown in FIG. 4C. This edge signal .DELTA.Y is limited by the limiting amplifier 16 and thus a non-correlation signal as shown in FIG. 4D is derived.
The level control circuit 17 carries out the control operation so that when the envelope-detected output of the carrier chrominance signal is large in level, the output level becomes large in correspondence therewith. Accordingly, when the color picture image contains, for example, much color components and the amplitude of the luminance signal is small, the amount of the edge signal .DELTA.Y subtracted from the luminance signal Y in the subtracting circuit 18 is increased. As a result, a luminance signal Y in which the edge portion is deteriorated as shown in FIG. 4E is produced at the terminal 8, thus resulting in poor reproducibility of the edge portion of the luminance signal and the picture image having an obscure edge.